The present invention relates to a method of producing a molded resin article. More particularly, the present invention relates to a method of producing a molded resin article composed of a mixture of thermoplastic and thermosetting materials.
In addition, the present invention relates to a method of fabricating a molded article from a composite material created by melting a thermosetting resin component and a thermoplastic resin component, kneading the components and immediately injecting the resultant composite material into a mold where the kneaded melt is given a desired shape.
Resins can be molded by various methods depending upon such factors as the physical properties of feed materials and those of the finished article. Thermoplastic resins are in most cases shaped by injection molding, in which a molten resin is injected at high pressure into a cavity between closed mold parts and given a desired shape. The injection molding process has a high production rate and great latitude in the selection of the shapes of molded articles.
The injection molding technique, however, has the following disadvantages: (1) a large clamping force must be used during molding and thus a bulky molding machine is required; (2) it is practically impossible to fabricate thin-Walled and large parts; and (3) the molten resin must have high temperature and pressure when it is injected into the mold cavity, such that simultaneous lamination of skin members such as fabrics and PVC leather cannot be achieved without high risk of damage to the texture of the skin members. The third problem is particularly pronounced in the case where napped fabrics are to be simultaneously laminated and flattening of naps is unavoidable. Electronic parts are often required to be sealed with resins. However, if injection molding is adopted to encapsulate the parts, displacement of parts, breaking of lead wires and other problems can occur owing to the high injection pressure employed. Furthermore, when molding resins are used as sealants of electronic parts, the resins are generally required to be fire-retardant. However, many of the fire-retardant resins available today have a tendency to decompose or deteriorate at elevated temperatures required for injection. Therefore, simultaneous lamination of skin members and resin molding of electronic parts are very difficult to accomplish by customary injection molding techniques. A fourth problem with the injection molding process is that since the resin material will pass through the whole length of the cylinder of a kneader at high speed under high pressure, long glass fibers cannot be incorporated without breaking into short lengths. In addition, use of a relatively high amount of glass fibers or inorganic fillers is difficult.
In order to cope with these problems, one needs to employ resin materials having a high melt index and inject them while in molten state into the mold cavity at low temperature and pressure, with the viscosity of the melt being reduced at the time of injection. In the case of thermoplastic resins, materials of high melt index are polymers of low molecular weights but articles molded from such low-molecular weight polymers are not satisfactory in many aspects including impact strength, fatigue strength, creep resistance, chemical and solvent resistance, and resistance to environmental stress cracking. In other words, in order to ensure that a resin feed solely composed of thermoplastic materials is injection molded at low temperature and pressure, the strength of the shaped article has to be sacrificed but such a sacrifice is not desired from a practical viewpoint.
Compared to thermoplastics, thermosets are low in viscosity when they are in a molten state but as a thermosetting reaction proceeds, they will cure and thus increase in viscosity. Therefore, to perform injection molding with an in-line machine, one of the following methods must be used: i) curing the resin by heating it after it has been injected into the mold cavity rather than in the injection cylinder; or ii) using a so-called "premix" prepared by compounding the ingredients after they have passed through a certain degree of reaction. However, the first method in which the resin is cured by heating in the mold cavity suffers from the disadvantage of extended molding cycles. The second method which involves the use of a premix is subject to significant limitations on such factors as the compounding formulation and the choice of starting materials.
A method that is commonly adopted in molding thermosetting resin materials having the problems described above includes first preparing a sheet molding compound (SMC) or bulk molding compound (BMC) which has a thermoset of interest, say, an unsaturated polyester impregnated in a glass mat or glass fibers, and then molding SMC or BMC by compression or with matched dies. However, this method is not suitable for high-volume production since the metering and supply of materials is not easily adaptable for automation or continuous processes.
Thermoplastic resins, typically polyurethanes, are often molded by reaction-injection molding (RIM) processes in which two liquid streams are pumped under high pressure into an impingement chamber where they are mixed intimately and then are immediately forced into a mold cavity where a rapid polymerization reaction occurs. However, the major disadvantage of the RIM process is a small freedom in the choice of compounding formulas because the materials that can be used are limited to those which are low in viscosity and high in reactivity. Furthermore, the viscosity of the resin is so low at the time of injection as to cause various troubles such as an increased chance of burring and formation of voids on account of bubble trapping, extensive bleeding in the bulk of fabrics to be laminated simultaneously, and difficulty in relatively highly loading reinforcements or fillers on account of the great tendency toward solid-liquid separation.
A possible alternative to injection molding of thermoplastics is stamping, in which a molten material is deposited onto an open matched mold and stamped by closing the mold. During stamping, the material flows, filling the mold cavity. Compared to injection molding, this method tolerates the use of low pressures and is suitable for fabricating large and thin-walled parts. However, so long as thermoplastics are used, there are limits on the effort toward temperature and viscosity reduction and the stamping method is still insufficient to completely eliminate the defects of the injection molding process.
As described above, all conventional methods for molding thermoplastic or thermosetting resins have their own problems and generally speaking, thermoplastics are adapted for high production processes. Another advantage of thermoplastics is that they can be molded into articles having a higher degree of toughness. In contrast, articles molded from thermosetting resins are almost brittle and prone to chipping but they are excellent in such properties as stiffness, heat resistance, stress cracking resistance and creep resistance. It is therefore expected that a molded article that is superior in production rate and physical properties will be obtained from thermoplastic and thermosetting resin materials that are blended in an appropriate composite form.
However, none of the molding methods currently in commercial use are capable of producing composites of desired thermoplastic and thermosetting resin materials in desired proportions. One of the practices that has been adapted extensively is to incorporate thermoplastic components as modifiers in SMC or BMC of unsaturated polyesters but the thermoplastic components that can be added have been limited to those which are soluble in the principal components of thermosets such as alkyds and styrene monomers. Besides this restricts compounding formulation, i.e. the contents of thermoplastic components that can be incorporated have been limited.
An attempt has also been made to fabricate thermoplastic resins by injection molding on an in-line machine after thermosetting components have been added. But in this case, too, the contents of thermosetting components that can be added are limited to low levels because if they are excessive, they undergo a curing reaction and the resulting mix becomes too viscous to be efficiently injected into the mold cavity.
As will be understood from the foregoing discussion, when thermoplastic and thermosetting resin materials are to be mixed and molded by previously known methods, either one type of resin serves as the principal component with the other being used merely as an additive, and it has been impossible for the two types of resins to be mixed in more or less comparable amounts, e.g. from 25:75 to 75:25, and directly subjected to a molding step.
In order to realize fabrication of large and thin-walled parts, simultaneous lamination of skin members, resin molding of electronic parts, high loading of glass fibers, or incorporation of long fibers, the molten material must be injected into a mold cavity not only at an optimum viscosity (e.g. lower than the viscosity encountered in ordinary injection molding of thermoplastics but higher than the viscosity encountered in RIM but also at low temperature and pressure. However, it has been impossible with conventional molding processes to accomplish the necessary adjustment or control of resin viscosity and temperature.
It is further noted that in order to attain a balance of high-level physical properties in a composite material, one of the ideal structures is a micro dispersion in which islands of a toughness imparting component are dispersed in a sea of a stiff and heat-resistant matrix. This ideal structure would be obtained by using a thermosetting resin as a component for stiffness and heat resistance of the matrix, and employing a thermoplastic resin as a component for toughness of the islands. It is desirable to use a rubber-like material as the thermoplastic resin component.
Conventional composite materials that are prepared by kneading two types of resin components, thermosets and thermoplastics, are characterized in that the thermoplastic resin component or rubber component is dispersed in a minor proportion in the thermosetting resin component. The rubber component is dissolved in the thermosetting resin component and when the solution changes from a fluid to a solid state upon reaction, the dissolved rubber component undergoes phase separation to form an emulsion in which the islands of the rubber component are microscopically dispersed in the sea of the thermosetting resin component. However, with conventional composite materials, the solubility of the rubber component is limited and it cannot be dissolved in an increased amount in the thermosetting resin component. Furthermore, the viscosity of the thermosetting resin component cannot be controlled with sufficient ease to enable efficient molding with customary machines including injection molding and extrusion machines. Even if a composite material in which the thermoplastic resin component and the thermosetting resin component are mixed in more or less comparable proportions is successfully molded, the islands of the thermoplastic resin component are not sufficiently dispersed microscopically in the sea of the thermosetting resin component to produce a molded article that attains a balance between stiffness, heat resistance and toughness at high levels. The following three principal reasons are offered to explain this problem: first, because of the nature of rubbery materials which form islands and are responsible for the toughness of the composite, considerable difficulty is involved in comminuting (pulverizing) the rubbery material into fine particles. Even if this is possible, the particles are so sticky as to experience frequent blocking, which makes it difficult to attain a particle size that is ideal for the formation of a desired dispersion; second, the island phase should generally be composed of particles on the submicron order but such fine particles tend to scatter as dust particles and this is detrimental not only to handling but also to the consistency of the supply of the rubber component through a feeder, etc.; and third, the rubber-like particles dispersed in the sea of the matrix resin will re-fuse during melt mixing under thermal effects, making it impossible to obtain an ideal dispersion.